Method of manufacturing an aluminum base photographic surface



United States Patent No Drawing.

This invention relates to an aluminum base photographic surface and method of making the same, and has as its primary object the provision of an improved aluminum base photo sensitive plate characterized by the fact that the same may be stored indefinitely in darkness without fogging of the light-sensitive materials therein and which, further, may be immediately developed by conventional photographic techniques without the necessity of additional treatment prior to development.

A further object of the invention is the provision of an improved method of manufacturing photographic plates which is materially lower in cost than hitherto known processes. Due to an improved technique of deposition, the percentage of silver salts required is substantially less, and as a result, production costs are reduced.

Still another object resides in the provision of an improved method of manufacturing such plates wherein a fine degree of control over the quantity of silver deposited may be maintained, thus producing more uniform and more predictable results, and hence a high degree of quality control.

A further object of the invention resides in the provision of a method wherein the necessity for the use of gelatin, as hitherto employed for a somewhat similar purpose, is completely obviated.

As conducive to a clearer understanding of this invention, it may here be pointed out that there are many known practices whereby the surfaces of aluminum or aluminum base alloys are first treated so as to render them porous and to subsequently chemically treat these surfaces to produce a light-sensitive medium in which the light-sensitive salts are bound in this porous surface. After such treatment, images can then be reproduced utilizing conventional methods known to the photographic industry. In the past, photographic images thus produced have presented two major drawbacks; one, that of providing an anodized surface which is both adherent and of sufficient pore size and density per unit area to absorb the light-sensitive halides in sufiicient volume; and secend, after having acquired such a layer they were then faced with the problem of depositing a silver halide salt such as silver chloride, silver bromide, silver iodide, or mixtures of these in the oxide coating while having this chemical deposition remain free from objectionable end results such as a blotching or spotty surface, a too rapid diffusion of the water solution salts, overall fogging of the sensitized surface and other deleterious eifects.

In the past, producers of photographic aluminum plates have restricted themselves to producing tightly adherent and highly absorbent oxide coatings without the minute exposed aluminum areas at the base of the pores. They have produced these layers by varying procedures within wide ranges of temperatures, current densities, and concentrations of the electrolyte. Some have even exposed the aluminum sheet first to electrolytic treatment containing chromic acid and then dilute solutions of oxalic acid as the electrolyte. Although these treatments in many cases increase pore density per unit area and in some cases the actual pore size, it has no effect on the barrier layer formed at the base of these pores on the inner face next to the aluminum. This barrier layer, although inert in characteristic, is infinitely thin and does not possess dielectric characteristics. By nature of its thinness, it is susceptible to voltage breakdown and is 3,321,385 Patented May 23, 1967 highly susceptible to corrosive chemicals. These characteristics manifest themselves in that upon exposure of an aluminum sheet in an electrolytic cell, after the initial peaking of the current and subsequent stabilization of this current density, current will continue to flow within the cell indefinitely. Further, in order to produce a barrier layer of sufficient density and uniformity, no provision is made, or provided for, with conventional methods of agitation to uniformly disperse the minute gas bubbles formed at the base of pores during oxidation.

If this inert barrier is not provided, it is found that when the plate is processed so as to deposit the light sensitive halides within the pores the ensuing results render the process useless.

Heretofore certain plates have been manufactured in a state wherein aluminum base photographic plates are provided which are not sensitive to light, and placed on the market in this state. These must be subsequently treated by the consumer immediately prior to use.

An important object of this invention is, therefore, the overcoming of all of these difliculties, and others, as will become apparent as the description of the invention proceeds.

The method of the instant invention contemplates oxidizing or depositing an aluminum oxide coating on the sheet of metal in question by first subjecting the sheet to an oxidizing process using any of the corrosive acids as an electrolyte. These acids could be sulfuric, oxalic, chromic, or any combination of these or any combinations of steps using these acids. As a source of current, either a DC. or an AC. potential may be used. The conditions of the anodizing bath with respect to temperature, concentration of acid, and time of anodization must be controlled. The acids previously mentioned are standard or at least have been used in the field of anodizing and are corrosive acids. That is, during anodizing a condition exists whereby while the aluminum oxide is being formed the corrosive electrolyte is simultaneously corroding some of this layer. By this principle, the coating is made porous. Obviously, the rate of formation as opposed to the rate of corrosion is the factor that determines the characteristics of the coating. In the instant method, after the preliminary steps of anodically coating the aluminum sheet, the sheet is exposed to an additional anodizing in a solution of boric acid. Boric acid in an electrolytic cell has the property of forming a non-porous aluminum oxide on the specimen which is noncorrosive in the electrolyte. Therefore, when the previously anodized specimen of aluminum has been anodized and then subsequently exposed to further oxidation in an electrolytic cell containing boric acid as the electrolyte, any metallic aluminum exposed at the base of the pores is further oxidized by this treatment. The aluminum oxide thus rapidly formed grows to considerable thickness and can be proved experimentally in that with electrolytes such as boric acid or mixtures of boric acid and borates the coating forms rapidly and the flow of current is soon reduced to substantially zero indicating that the growth of the coating has ceased since contact of electrolyte and metal no longer exists. Such a coating is virtually insoluble in the electrolyte and is impervious and nonabsorptive. Whereas the growth of a coating in an electrolyte such as boric acid stops when a definite thickness is reached, coatings made in electrolytes such as sulfuric acid, oxalic acid, chromic acid, and the like, continue to grow in thickness as long as a suitable potential is applied. Contrary to the impervious type of film, the films formed in an electrolyte in which the oxide formed is appreciably soluble, pores continue to form and as long as the current continues to flow, the electrolyte must penetrate to the metal surface through these pores. By virtue of the subsequent treatment using boric acid as the electrolyte, a substantial barrier layer is formed, as evidenced by the fact that if this barrier layer did not exist in sulficient thickness there would be a continued flow of current.

In the anodic oxidation of aluminum, regardless of the conditions maintained or the electrolytes used, it is necessary to maintain this electrolyte in a constant state of excitation. If this precaution is not taken, the tiny gas bubbles formed at the base of the pores which subsequently find their way up through the pores and out into the solution and are subsequently released as vapor, have a tendency to remain in the pores and also because of surface tension cling to the face of the specimen being treated. It is therefore necessary in practice to constantly agitate the electrolyte. This agitation causes the gas bubbles to break free of the surface being anodized allowing a more even attack of the electrolyte upon the anodized surface. This agitation helps also in some degree to partially remove, or rather allow the small bubbles formed within the pores to be released.

Normally, the microscopic bubbles of oxygen at the base of the pores are released intermittently, or in some cases not at all. Unless the microscopic bubbles are removed, and removed simultaneously, a non-uniform attack of the electrolyte upon the surface of the metal results. This condition affects the uniformity of the surface, the porous structure, and most of all, the formation of the barrier layer on the inner face of the aluminum. Conventional methods of agitation are accomplished by either mechanically stirring the electrolyte causing some pattern of motion, or more often by the immersion of perforated pipes introduced at the bottom of the anodizing tanks through which air pressure is released periodically. In the present method, during the process of anodically coating the aluminum sheet, in addition to and augmenting the use of agitation using compressed air in the anodizing tank, a transducer assembly powered by an ultrasonic generator in the range of 25 kc. with a wattage output in the neighborhood of 300 watts is introduced into the tank, which provides violent cavitation of the electrolyte. This can be described as a cold boil and can be visualized as millions of powerful fingers of energy playing upon the surface being treated which act to break up the gas bubbles generated in the cell. There is little or no restriction as to frequency of oscillation or power output. A wide range of frequencies and power output is available in commercial units. The suitable choice of one is determined by the way in which a liquid responds to ultrasonic agitation as applied to this particular application. The practical range for this application has been found to be from approximately 25 kc. (below this frequency there is the discomfort to personnel on account of audible squeals) to about 40 kc. Selection of a specific unit depends largely on the volume of solution or an electrolyte agitated and the resonance of the systern. Ultrasonic cavitation as applied to the instant method of anodizing enables a uniform and complete release of the microscopic bubbles of gas trapped at the base of the pores thereby providing an extremely uniform oxidized layer and permitting a uniform and maximum buildup of a barrier layer at the base of the pores. The auxiliary use of air released through the solution as a means of agitation is employed since this anodic coating is composed of a spongy or porous layer containing millions of pores per unit area. In the use of ultrasonic cavitation as a means of agitation in cases where there exist deep channels or blind holes such as the pores that can entrap air or gases, it is necessary to set in motion either the aluminum sheet or the electrolyte in order to permit the air or gas to escape. If there is no liquid in contact with the surface of the metal, there can be no cavitation. After washing and drying, the prepared oxidized aluminum surface is immersed in a solution consisting of an organic compound capable of reducing silver ions. It is advantageous to use an organic dye which has an afiinity to aluminum oxide and possesses the characteristics of a mordant.

Summarized, the method of the instant invention consists of forming upon a sheet of pure aluminum a tightly adherent and highly absorbent oxide coating. Such coatings are not only hard and adherent, but contain a pore structure of sufiicient pore size and density such as to be highly absorbent to the various aqueous solutions in subsequent treatments. For the initial oxidizing of the surface, the aluminum sheet is treated in an anodizing bath electrolytically or with an oxidizing agent in solution. In treating it electrolytically, the aluminum sheet is anodized in a cell, using as an electrolyte a mixture of oxalic acid and oxalates in a pH range of l to 6, with a current density of approximately .080 ampere per square inch. It is thus exposed for from 45 to 60 minutes. After rinsing and the subsequent neutralizing of any residual acid in the coating thus formed, the sheet is then exposed in an electrolytic cell utilizing boric acid as the electrolyte. The boric acid is maintained in the pH range of 1 to 6 with a current density of approximately .100 ampere per square inch. This treatment is continued for from to minutes or until the current flowing through the cell reaches substantially zero. The thickness of the barrier layer thus formed is proportional to the impressed voltage across the cell. In both the preliminary and secondary treatments, the electrolyte is kept in a state of agitation by the intermittent release of compressed air. In addition to this, an ultrasonic energy wave is introduced of approximately .75 watt per square inch in the order of to kc. As a result, there is formed upon the aluminum sheet a highly porous adherent surface and in addition there is formed upon the inner face of this coating a heavy barrier layer of sufficient thickness as to prevent contact of light-sensitive salts with metallic aluminum. The aluminum sheet is then immersed in an aqueous dyestutf which is both a reducing agent and a mordant, the only requisite being that the dyes used have a chemical afiinity to aluminum oxide, and to be of an organic nature in order to reduce the aqueous silver salts, and to act as a mordant. Silver can then be precipitated in the layer through chemical decomposition by means of a solution containing aqueous silver salts. Many dyestuffs capable of reducing silver ions can be readily embedded in the oxide layer down to the base of the pores, either by simple impregnation in the form of more or less concentrated solutions in water or other solvents, or by their actual absorption through chemical or physical procedures. Decomposition of the dyestufi embedded in the oxide layer with silver containing solutions is advantageously carried out by immersing the pre-treated aluminum sheet in the reactive silver solution. It is preferred to have the latter in a pH range of from 4 to 6. Ammoniacal silver solutions, preferably those having no or only a slight excess of ammonia, have been found to be suitable. The temperature at which this immersion takes place and the time required for it depend on the nature of the layer and the kind of dyestuff used. Preferably a dyestutf should be selected, the oxidation products of which can be dissolved out from the absorbent layer so that nothing of it remains in the layer. By the process described, it becomes possible to precipitate metallic silver in the oxide layer in sufiicient quantity and in a depth down to the base of the pores. By this process, a certain portion of the silver remains combined with the dyestulf in a complex compound. Use of a dye as a reducing agent in lieu of known colorless reducing agents such as a pyrogallic acid has the advantage in that the silver thus precipitated may be controlled by the depth of the color assumed by the layer. Such a control of silver precipitated permits a standardized output of photographic or copying material which as of necessity must be of constantly uniform quality and not have any variations when processed; otherwise, in subsequent processing there would be inevitable differences of intensity in the final product. These steps may be carried out in daylight.

The metallic silver precipitated thusly in the layer is present in an ultra-microscopically fine state, from which it can be readily re-oxidized to a silver compound and is then easily converted into a photosensitive silver halide by immersing the specimen in a solution containing at least one oxidizing agent and one or more halides. If silver chloride has been produced by this method it can be converted wholly or partially into the corresponding bromide or iodide by treatment with solutions of bromides or iodides. In this way it is possible to vary photosensitivity within wide limits. After formation of the lightsensitive halides within the coating, the sheet of aluminum is further treated in suitable ripening solutions. The aluminum sheets so prepared can then be used for photographic purposes such as photographic plates, film or paper. They may be exposed beneath a suitable photographic negative and developed in a mineral or organic developer. The images so produced may be toned, intensified, reduced and fixed in the known manner. It is also possible to seal this surface by immersing the aluminum sheet in a solution of distilled water at 100 C. for approximately 30 minutes thus aluminum monoh drates are formed closing the pores of the coating, or by immersion in hot solutions of hydrolytic salts, such as nickel or cobalt salts or by treating with steam.

Specific examples of a method of producing an aluminum base sheet of photographic material in accordance with the instant invention are as follows:

Example No. 1: After conventional pretreatment, a sheet of pure aluminum is anodically treated with a current density of .080 ampere per square inch in a solution of oxalic acid and oxalates at a temperature of 45 C. using a direct current for 60 minutes. The solution is made up by dissolving 250 grams of potassium oxalate in liters of distilled water. Oxalie acid is then added and dissolved therein until the solution has a hydrogen exponent of pH 2.5. The electrolyte is agitated and cavitation is introduced as described above. The sheet thus treated is thoroughly rinsed in distilled water and subjected to further anodic treatment in an electrolytic cell containing a boric acid electrolyte in the order of a 5 percent solution. It is thus anodically treated for a period of minutes at a current density of .100 ampere per square inch while the temperature is maintained at 50 C. During this treatment the electrolyte is again agitated and cavitation is introduced as above. After the anodic coating has been formed, the aluminum sheet is then removed from the cell, thoroughly rinsed, and further subjected to a 5 percent solution of bicarbonate of soda. This step is for the purpose of neutralizing any residual acid in the coating. The aluminum sheet is then rinsed in distilled water and dried in a drying cabinet at an elevated temperature in the order of 50 C. The aluminum sheet thus prepared, is then immersed in an aqueous solution of 2,4-dichlorol-l-napthol. In order to standardize this dye coupling technique, a stock solution of 5 percent was used adjusting the pH to 6.8. The temperature was maintained at 45 C., plus or minus one degree, and maintained in this solution for fifteen minutes. A uniform medium blue color is thus imparted to the oxide film. By maintaining a rigid control of the coupling solution and the subsequent matching of color so produced against a standard color chart, the precipitation of metallic silver may be controlled to a high degree. The sheet of aluminum is then removed from the dye bath, rinsed and immersed while still wet in a 5 percent solution of silver nitrate having a slight excess of ammonia added. It is kept in this solution at a temperature of 45 C. for sixty minutes. Upon removal from the silver nitrate solution, the sheet is then washed for a short period of time in distilled water and then dried at a temperature of 50 C. An anodic coating is thus deposited on the sheet of aluminum, in which there has been formed an impervious barrier layer and into whose pores have been precipitated finely divided metallic silver. Subsequent steps of sensitizing are then carried out in a darkroom. The sheet of aluminum is now immersed in a solution made up of 3 grams of potassium pennanganate, 6.25 grams of potassium ferricyanide dis-solved in 500 cos. of distilled water. It is allowed to remain in this solution for five minutes at a temperature of 18 to 20 C. This solution is of a strong oxidizing nature which serves to oxidize the metallic silver which was precipitated in the layer and to bleach out any dye which may be present. In this bath the plate Will change from a black to a light brown color. The plate is then rinsed in distilled water and is immersed in a solution made up of the following: 10 grams of oxalic acid, 6.25 grams of potassium bromide, 3 grams of sodium chloride, .025 gram of ammonium iodide all dissolved in 500 cos. of distil'ed water. It is by this step that light-sensitive halides are formed and the sheet of aluminum takes on a whitish coloration. After rinsing in distilled water for approximately five minutes, the sheet of aluminum is then treated in a solution of 9 grams of potassium ferricyanide and 3.5 grams of potassium bromide per liter. The thorough- 1y washed plate is then force dried at approximately 60 C. and stored in the dark until ready for use. Under conditions normal to the storing of photographic material, these plates or plate so treated will have an indefinite shelf-life. They can later be processed by methods commonly used in the photographic industry.

Example 2: Same as Example 1 except that a further ripening solution consists of a mixture of 1 percent potassium bromide and 1 percent potassium iodide.

Example 3: Same as Example 1 except that the ripening solution contains 5 minirns of silver nitrate.

From the foregoing it will now be seen that there is herein provided an improved photographic plate and method of making the same which accomplishes all of the objects of this invention, and others, including many advantages of great practical utility and commercial importance.

As many embodiments may be made in this inventive concept, and as many modifications may be made in the embodiments herein shown and described, it is to be understood that all matter herein is to be interpreted merely as illustrative and not in a limiting sense.

I claim:

1. A method of producing an aluminum photographic sheet which comprises the step of subjecting a sheet of pure aluminum to anodic treatment in a solution of oxalic acid and oxalates made by dissolving 250 grams of potassium oxalate in 10 liters of distilled water with added oxalic acid to a hydrogen exponent of pH l6 with a current density of .080 ampere per square inch for 45 to 60 minutes at a temperature of approximately 45 C., agitating the electrolyte during such treatment by passing air therethrough, additionally agitating the electrolyte by ultrasonic vibration of approximately .75 watt per square inch at to 40 kc., rinsing the sheet in distilled water, further anodically treating the sheet in an electrolyte solution of 5 percent boric acid of pH 16 for a period of 15 to 20 minutes at a current density of .100 ampere per square inch at a temperature of approximately 50 C. while agitating the electrolyte by the passage of air therethrough and additionally by ultrasonic agitation, rinsing the sheet, immersing the sheet in a 5 percent solution of bicarbonate of soda to neutralize any residual acid, again rinsing the sheet in distilled water, drying the sheet at a temperature of approximately 50 C., immersing the sheet in an aqueous solution of 5 percent 2,4-dichlorol-lnapthol adjusted to a pH of 6.8, at a temperature of approximately C. for a period of approximately fifteen minutes, again rinsing the sheet, immersing the sheet While still wet in a 5 percent solution of silver nitrate having a slight excess of ammonia added for approximately sixty minutes, rinsing the sheet in distilled water and drying the same at a temperature of approximately C., immersing the sheet in a solution of 3 grams of potassium permanganate and 6.25 grams of potassium ferricyanide dissolved in 500 ccs. of distilled water for a period of five minutes at a temperature of from 18 C. to 20 C. to oxidize the precipitated metallic silver, rinsing the plate and immersing the same in a ripening solution comprised of 10 grams of oxalic acid, 6.25 grams of potassium bromide, 3 grams of sodium chloride and .025 gram of ammonium iodide dissolved in 500 cos. of distilled water, to form light-sensitive halides, rinsing in distilled water and immersing the sheet in a solution of 9 grams of potassium ferricyanide and 3.5 grams of potassium bromide per liter of distilled Water, and finally force drying the plate at approximately 60 C.

2. A method of producing an aluminum photographic sheet which comprises the step of subjecting a sheet of pure aluminum to anodic treatment in a solution of oxalic acid and oxalates with a current density of .080 ampere per square inch for approximately sixty minutes at a temperature of approximately 45 C., agitating the electrolyte during such treatment by passing air therethrough, additionally agitating the electrolyte by ultrasonic vibration, rinsing the sheet in distilled water, further anodically treating the sheet in an electrolyte solution of percent boric acid for a period of fifteen minutes at a temperature of approximately 50 C. While agitating the electrolyte by the passage of air therethrough and additionally by ultrasonic agiitation, rinsing the sheet, immersing the sheet in a 5 percent solution of bicarbonate of soda to neutralize any residual acid, again rinsing the sheet in distilled water, drying the sheet at a temperature of approximately 50 C., immersing the sheet in an aqueous solution of 5 percent 2,4-dichlorol-1-napthol for a period of approximately fifteen minutes, again rinsing the sheet, immersing the sheet while still wet in a 5 percent solution of silver nitrate having a slight excess of ammonia added for approximately sixty minutes, rinsing the sheet in dis tilled water and drying the same at a temperature of approximately 50 C., immersing the sheet in a solution of potassium permanganate and potassium ferricyanide dissolved in distilled water for a period of five minutes at a temperature of from 18 C. to 20 C. to oxidize the precipitated metallic silver, rinsing the plate and immersing the same in a ripening solution comprised of oxalic acid, potassium bromide, sodium chloride and ammonium iodide dissolved in distilled water, to form light-sensitive halides, rinsing in distilled Water and immersing the sheet in a solution of potassium ferricyanide and potassium bromide in distilled Water, and finally force drying the plate at approximately C.

3. The method of claim 1 wherein the plate is immersed in a further ripeningsolution consisting of 1 percent potassium bromide and 1 percent potassium iodide before final drying.

4. The method of claim 2 wherein 5 minims of silver nitrate are added to the ripening solution.

References Cited by the Examiner UNITED STATES PATENTS 2,115,339 4/1938 Mason 96-86 2,126,017 8/1938 Jenny et al. 96-86 2,448,513 9/1948 Brennan et al. 20435 2,647,079 7/1953 Burnham 20438 2,687,373 8/1954 Hering 20438 2,710,804 6/1955 Schenk 20438 2,766,119 10/1956 Freedman et al.

2,861,932 11/1958 Pohl 204273 2,919,235 12/1959 Roller 20496 3,039,951 6/1962 Clenard et al. 204-273 3,148,057 9/1964 Raether 961 OTHER REFERENCES Wernick, Finishing of Aluminum, 1959; Draper Ltd.; pp. 220-437, 1959.

JOHN H. MACK, Primary Examiner.

MURRAY TILLMAN, Examiner.

L. G. \VISE, W. VAN SISE, Assistant Examiners. 

1. A METHOD OF PRODUCING AN ALUMINUM PHOTOGRAPHIC SHEET WHICH COMPRISES THE STEP OF SUBJECTING A SHEET OF PURE ALUMINUM TO ANODIC TREATMENT IN A SOLUTION OF OXALIC ACID AND OXALATES MADE BY DISSOLVING 250 GRAMS OF POTASSIUM OXALATE IN 10 LITERS OF DISTILLED WATER WITH ADDED OXALIC ACID TO A HYDROGEN EXPONENT OF PH 1-6 WITH A CURRENT DENSITY OF 9/,/ AMPERE PER SQUARE INCH FOR 45 TO 60 MINUTES AT A TEMPERATURE OF APPROXIMATELY 45%C., AGITATING THE ELECTROLYTE DURING SUCH TREATMENT PASSING AIR THERETHROUGH, ADDITIONALLY AGITATING THE ELECTROLYTE BY ULTRASONIC VIBRATION OF APPROXIMATELY .75 WATT PER SQUARE INCH AT 25 TO 40 KC., RINSING THE SHEET IN DISTILLED WATER, FURTHER ANODICALLY TREATING THE SHEET IN AN ELECTROLYTE SOLUTION OF 5 PERCENT BORIC ACID OF &'' 1-6 FOR A PERIOD OF 15 TO 20 MINUTES AT A CURRENT DENSITY OF .100 AMPERE PER SQUARE INCH AT A TEMPERATURE OF APPROXIMATELY 50%C. WHILE AGITATING THE ELECTROLYTE BY THE PASSAGE OF AIR THERETHROUGH AND ADDITIONALLY BY ULTRASONIC AGITATION, RINSING THE SHEET, IMMERSING THE SHEET IN A 5 PERCENT SOLUTION OF BICARBONATE OF SODA TO NEUTRALIZE ANY RESIDUAL ACID, AGAIN RINSING THE SHEET IN DISTILLED WATER, DRYING THE SHEET AT A TEMPERATURE OF APPROXIMATELY 50%C., IMMERSING THE SHEET IN AN AQUEOUS SOLUTION OF 5 PERCENT 2,4-DICHLOROL-1NAPTHOL ADJUSTED TO A PH OF 6.8, AT A TEMPERATURE OF APPROXIMATELY 45%C. FOR A PERIOD OF APPROXIMATELY FIFTEEN MINUTES, AGAIN RINSING THE SHEET, IMMERSING THE SHEET WHILE STILL WET IN A 5 PERCENT SOLUTION OF SILVER NITRATE HAVING A SLIGHT EXCESS OF AMMONIA ADDED FOR APPROXIMATELY SIXTY MINUTES, RINSING THE SHEET IN DISTILLED WATER AND DRYING THE SAME AT A TEMPERATURE OF APPROXIMATELY 50%C., IMMERSING THE SHEET IN A SOLUTION OF 3 GRAMS OF POTASSIUM PERMANGANATE AND 6.25 GRAMS OF POTASSIUM FERRICYANIDE DISSOLVED IN 500 CCS. OF DISTILLED WATER FOR A PERIOD OF FIVE MINUTES AT A TEMPERATURE OF FROM 18%C. TO 20*C. TO OXIDIZE THE PRECIPITATED METALLIC SILVER, RINSING THE PLATE AND IMMERSING THE SAME IN A RIPENING SOLUTION COMPRISED OF 10 GRAMS OF OXALIC ACID, 6.25 GRAMS OF POTASSIUM BROMIDE, 3 GRAMS OF SODIUM CHLORIDE AND .025 GRAM OF AMMONIUM IODIDE DISSOLVED IN 500 CCS. OF DISTILLED WATER, TO FORM LIGHT-SENSITIVE HALIDES, RINSING IN DISTILLED WATER AND IMMERSING THE SHEET IN A SOLUTION OF 9 GRAMS OF POTASSIUM FERRICYANIDE AND 3.5 GRAMS OF POTASSIUM BROMIDE PER LITER OF DISTILLED WATER, AND FINALLY FORCE DRYING THE PLATE AT APPROXIMATELY 60%C. 